Multi-layer carbonless sheet product

ABSTRACT

A magnetically-activatable sheet product is provided comprising a pair of laminated outer sheets between which is a magnetic layer comprising magnetically-activatable particles in a binder matrix, the outer sheets having sufficient opacity to mask the appearance of the magnetic layer, at least one of the outer sheets being provided on its outward facing surface with a coating which comprises either microcapsules containing a solution of at least one chromogenic material, or dispersed droplets containing at least one chromogenic material in a pressure-rupturable matrix, or a colour developer composition, or both microcapsules containing at least one chromogenic material and also a colour developer.

This invention relates to a multi-layer carbonless sheet product, one ofthe layers of which comprises magnetically-activatable particles in abinder matrix.

Various types of pressure-sensitive (so-called “carbonless”) copyingpaper are known, of which the most widely used is the transfer type. Abusiness forms set using the transfer type of pressure-sensitive copyingpaper comprises an upper sheet (usually known as a “CB” sheet) coated onits lower surface with microcapsules containing a solution in an oilsolvent or solvent composition of at least one chromogenic material anda lower sheet (usually known as a “CF” sheet) coated on its uppersurface with a colour developer composition. If more than one copy isrequired, one or more intermediate sheets (usually known as “CFB”sheets) are provided, each of which is coated on its lower surface withmicrocapsules and on its upper surface with colour developercomposition. Imaging pressure exerted on the sheets by writing, typingor impact printing ruptures the microcapsules, thereby releasing andtransferring chromogenic material solution on to the colour developercomposition and giving rise to a chemical reaction which develops thecolour of the chromogenic material and so produces a copy image. In avariant of the above-described arrangement, the solution of chromogenicmaterial may be present as dispersed droplets in a continuouspressure-rupturable matrix instead of being contained within discretepressure-rupturable microcapsules. In another type of pressure-sensitivecopying system usually known as a self-contained or autogenous system,microcapsules and colour developing co-reactant material are coated ontothe same surface of a sheet, and writing, typing or printing on a sheetplaced above the thus-coated sheet causes the microcapsules to ruptureand release the solution of chromogenic material, which then reacts withthe colour developing material on the sheet to produce a coloured image.

Such forms are generally used in applications involving an iterative orrepeated process in which various sheets are removed at various stagesin the process, often with additional written information, for example asignature or date, being added to one or more sheets. Loss or alterationof written data can cause a problem, and elaborate paper handlingsystems need to be in place to ensure proper tracking.

International (PCT) Patent Application No. WO 01/92961A discloses asheet material carrying a coating containing cavities in whichelectrically- and/or magnetically-activatable particles are located. Thethus-coated sheet material is machine-writable and -readable in asimilar manner to media such as audio or video tapes, and floppy andhard disks for use in computers. The magnetically-activatable particlesdisclosed in WO 01/92961A are of the same general kind as used in mediaas just referred to, and include chromium dioxide, iron oxide,polycrystalline nickel-cobalt alloys, cobalt-chromium or cobalt-samariumalloys, or barium-ferrite. The base sheet on which the particles arecoated is typically a natural or synthetic paper.

Such magnetically-activatable materials are strongly-coloured, forexample dark brown, dark grey or black. As a result, papers carryingcoatings of such materials have an aesthetically-unattractive appearancewhich does not match the excellent whiteness, opacity, printability andappearance normally expected of good quality paper products.

WPI Abstract Acc. No. 1989-244425 of JP 1176325 describes a magneticrecording paper with good printability which consists of a base paper, amagnetic recording layer formed on one side of the base paper, and acoating formed on the magnetic recording layer and made from an aqueouspaint containing a white pigment.

Our experience is that it is not readily possible to mask theunattractive appearance of the magnetic layer of such products by theprovision of a white-pigmented topcoat over the magnetically-activatablecoating, unless very high pigment coatweights are used, for example ofthe order of 20 g m⁻². Even with such coatweights, the appearance of thefinal product may not be fully satisfactory.

WPI Abstract Acc. No. 2000-649395 of JP 2000192398 describes a wallpapercomprising sheets of paper containing iron powder kneaded with paste inbetween, which can be affixed to a concrete wall or to plaster board.Pictures and posters can then be affixed to the wall using magnets.

UK Patent Application No. 2109302A describes a sheet material comprisinga three-layer laminate, the outer layers of the laminate being made ofpaper, and the middle layer of which is opaque. The middle layer may bea ferric oxide or magnetic oxide composition, and the specificationstates that sheet material in which a ferric oxide composite is presentin the middle layer has been found to be capable of retaining magneticimages similarly to recording tape.

None of the above prior art makes any mention of pressure-sensitivepapers.

The present invention provides a magnetically-activatable sheet productcomprising a pair of laminated outer sheets between which is a magneticlayer comprising magnetically-activatable particles in a binder matrix,the outer sheets having sufficient opacity to mask the appearance of themagnetic layer; at least one of the outer sheets being provided on itsoutward facing surface with a coating which comprises eithermicrocapsules containing a solution of at least one chromogenicmaterial, or dispersed droplets containing at least one chromogenicmaterial in a pressure-rupturable matrix, or a colour developercomposition, or both microcapsules containing at least one chromogenicmaterial and also a colour developer.

The outward facing surfaces may be coated with microcapsules containinga solution of at least one chromogenic material, or dispersed dropletscontaining at least one chromogenic material in a pressure-rupturablematrix on one side of the sheet and a colour developer composition onthe opposite side of the sheet. A combination of the described coatingscould also be used to manufacture self-contained CB sheets and the likecommonly used in the carbonless paper industry.

The sheet product according to the invention is a sheet which is apressure-sensitive copying system, or which comprises part of apressure-sensitive copying system. Thus the sheet may comprise a CFlayer, CB layer or autogenous layer via single coating. A CFB sheetcould comprise CB and CF coating layers applied to opposite sides of thesheet.

The invention also provides a method of storing digital magnetic data,which comprises writing digital data to a product according to theinvention using a magnetic data writer.

The invention also provides a method of reading digital magnetic data,which comprises writing digital data to a product according to theinvention using a magnetic data writer, and subsequently reading saiddata using a magnetic data reader.

The outer sheets are preferably of paper, although plastic sheetmaterials which simulate the properties of paper (so-called “syntheticpaper”) can alternatively be used.

The magnetic layer may be formed by a coating (hereafter referred to asa “magnetic coating”) on the inwardly facing surface of one or both ofthe outer sheets, or may be formulated as a laminating adhesive which isapplied as or just before the two outer sheets are brought together in alaminating press or similar equipment.

In a preferred embodiment of the invention, the or each (one or both)outer sheet carries a pigment/binder primer coat on its inward facingsurface. This enhances the masking effect, and also improves themagnetic properties of the product. It improves the “hold out”properties of the sheet with respect to a subsequently-applied magneticcoating, thereby facilitating the application of the magnetic coating ina uniform and even manner and minimizing waste of the magnetic coatingby absorption into the body of the sheet.

Conveniently, the outer sheets are substantially identical (other thanthe outer coating layer(s)) and each comprises a base sheet of naturalor synthetic paper, a pigment/binder primer coat and a magnetic coatingon top of the primer coat. When laminated, the magnetic coatings are inface to face contact, and together form a single central magnetic layer.Alternatively, the magnetic layer can be formed by a magnetic coatingpresent on only one of the two outer sheets. In either case, anadditional laminating binder or adhesive is normally used to secure thesheets together to form the laminate. Such a binder may be, for example,a polyvinyl alcohol, a latex, a starch or a proteinaceous binder such asa soy protein derivative. A still further possibility, as alreadymentioned, is for the magnetic coating to be formulated as a laminatingadhesive which is applied as or just before the two outer sheets arebrought together in a laminating press or similar equipment. Theadhesive or binder component of such an adhesive can be as justdescribed for a laminating adhesive not containingmagnetically-activatable particles.

The primer coat which may be present on one or both of the outer sheetsis typically formulated from conventional coating pigments as used inthe paper industry, for example calcium carbonate (particularlyprecipitated calcium carbonate), kaolin or other clays (particularlycalcined clays) and/or, where high opacity is required and justifies theextra cost, titanium dioxide. The binder used can be conventional, forexample a latex (particularly a styrene-butadiene or acrylic latex), astarch or starch derivative, a polyvinyl alcohol and/or a soy proteinderivative or other proteinaceous material. The primer coatweight istypically in the range of about 5 to 15 g m⁻², but this can vary inaccordance with the masking effect desired and the basis weight of theouter sheets used (heavier base papers normally require lower primercoatweights).

The magnetic coating can be formulated from magnetically-activatablematerials as already referred to, for example chromium dioxide, ironoxide, polycrystalline nickel-cobalt alloys, cobalt-chromium orcobalt-samarium alloys, or barium-ferrite, although these do notconstitute a comprehensive list of suitable materials. The binder usedcan be selected from the same materials as disclosed above for use inthe laminating adhesive, but is typically a styrene-butadiene or acrylicor other latex. The coatweight applied is typically such that up toabout 10 g m⁻² of magnetically-activatable material is present, but thiscan be varied in accordance with the level of magnetic characterrequired. The magnetic coating can if desired contain an extender suchas calcium carbonate, which not only offers cost reduction but alsohelps to reduce the darkness of the magnetic layer.

The material used for the outer sheets must be such as to provide asatisfactory masking effect and desirability also a good final productappearance, and capable of being visibly written or printed uponsatisfactorily, but otherwise can be chosen to suit the intended finaluse of the laminated sheet product. For example, the outer sheets may beof a lightweight base paper (typically about 50 g m⁻² or less), so thatwhen laminated, the final product will not be excessively thick orheavy. Lightweight base papers of the kind conventionally used inpressure-sensitive copying paper are of course particularly suitable inthis context, since they are of good appearance and combine lightnesswith strength. In general, an outer sheet will be regarded as havingsufficient coverage/opacity to mask the appearance of the magnetic layerif the whiteness of the resulting product, measured on an Elrepho 3000instrument with the use of UV light enhancement, is within 5 points ofthe original base sheet on the L scale. Preferably the whitenessapproaches that of the original base sheet used to produce the product.

Although it is possible to use a primer coat on each outer sheet toachieve the desired opacity, smoothness and hold-out for the outersheets of the final laminated product, uncoated papers can be used forone or both of the outer sheets. Where the product contains only oneouter sheet bearing an inward-facing primer coat, magnetic data ispreferably written to and read from the side of the product carrying theprimer coating.

Because of the nature of the processes in which pressure-sensitivepapers are commonly used, the ability of one or more sheets of a set tocarry magnetic information as well as visible information is a majoradvance, since it reduces or eliminates the requirement for humanintervention and/or the requirement for retention of data stored, forexample on business forms, in physical rather than electronic form. Thepresent invention enables sheets to be provided not only with visiblewritten information, but also with magnetically written information.This provides major benefits in terms of paper handling andconsequential lowering of costs, in numerous circumstances.

Pressure sensitive coatings containing microcapsules would normally butnot exclusively be coated following the lamination of the outer sheetsdescribed above, to prevent premature capsule rupture and impairedcarbonless image formation. Non-microcapsule containing coatings may becoated either before or after the lamination of the outer sheetsdescribed above, as pressures applied to these coatings do not influencetheir carbonless image forming characteristics.

A preferred example of a product according to the invention comprises afirst outer sheet provided with a pigment/binder primer coat on itsinward facing surface, a magnetic layer comprisingmagnetically-activatable particles in a binder matrix, and a secondouter sheet being provided on its outward facing surface with a coatingwhich comprises a colour developer composition. Such a sheet willprovide the bottom sheet of a set of business forms, and magnetic datacan be written and read onto the form from the bottom of the set.

The invention will now be illustrated by the following Examples, inwhich all parts and percentages are by weight unless otherwisespecified. FIGS. 1 to 6 illustrate products obtained in the Examples.Examples 1 to 3 illustrate basic techniques for producing a laminatedsheet including a magnetic layer, while Examples 4 to 10 illustratecarbonless products according to the invention.

EXAMPLE 1

The product formed in this example is illustrated in FIG. 1, in which(1) represents sheets of paper; (2) represents pigment/binder primercoats applied to the inward facing surfaces of sheets (1); and (3)represents a magnetic layer.

A 49 g m⁻² strong lightweight base paper of the kind conventionally usedin pressure-sensitive copying paper was blade coated on a large-scalepilot plant coater with a 46% solids content aqueous primer coatformulation of the following composition: Component Parts by weight (drybasis) Calcined clay 100 Oxidised potato starch 5 Styrene-butadienelatex 15

The coatweight applied was about 9 g m⁻² on a dry basis, and the resultwas an opaque paper with a flat primer-coated surface.

The primer coated surface was then coated with a 41% solids contentaqueous magnetic coating formulation using a small scale pilot plantblade coater. The coatweight applied was about 10 g m⁻² on a dry basis,and the coating formulation was as follows: Component Parts by weight(dry basis) Iron oxide 100 Styrene-butadiene latex 17.6

A small-scale pilot coater/laminating press was used to laminate one plyof primer- and magnetic-coated paper as just described to aprimer-coated sheet as described above but which did not carry amagnetic coating. The magnetic-coated surface faced inward, so that itformed a magnetic layer between the two paper plies. A 15% solidscontent aqueous solution of polyvinyl alcohol was used as a laminatingadhesive and was continuously rod coated on to the magnetic coating justbefore the laminating nip.

The resulting product was then magnetically imaged (encoded) with a barcode using inductive magnetic writing equipment of the kindconventionally used for encoding the magnetic strips of credit cards.The resulting magnetic image was found to be readable using a magneticloop reader or suitably configured oscilloscope.

EXAMPLE 2

This utilized two plies of the same primer-coated paper as in Example 1,but no magnetic coating was applied. Instead, a magnetically-activatableaqueous laminating adhesive formulation was applied by rod coating at arange of different application rates to one ply just before the twoplies were laminated in a pilot scale laminating press as described inExample 1.

The laminating adhesive formulation had the following composition on adry basis, and was applied at 50% solids content. Component Parts byweight (dry basis) Iron oxide 84 Calcium carbonate extender 16Styrene-butadiene latex 17

Six different application rates were applied ranging from 8 to 20 g m⁻²on a dry basis. This gave iron oxide contents of about 1 to 3 g m⁻².

All the resulting products were magnetically imageable and readable inthe same manner as described in Example 1.

EXAMPLE 3

This was a variant of Example 1 in which each of the two primer-coatedplies carried a magnetic coating, rather than just one. A lower magneticcoatweight was used (5 g m⁻²), so as to give much the same totalmagnetic layer coatweight. The composition of the magnetic coating wasas in Example 1.

EXAMPLES 4 TO 6

These examples illustrate various methods for the production of a sheetof “carbonless” paper carrying a CF layer. The products formed areillustrated in FIG. 2, in which (1) represents sheets of paper; (2)represents pigment/binder primer coats applied to the inward facingsurfaces of sheets (1); (3) represents a magnetic layer; and (4)represents a CF layer.

EXAMPLE 4

In this example, the magnetic laminate product was converted into acarbonless CF (coated front) product via coating following thelamination process.

Magnetic laminate product, of the type described in Example 1, wascoated with a 50% solids content clay based CF coating formulation usinga laboratory Meyer bar rod coater to obtain a coatweight of between 5-10g/m² CF on a dry weight basis. The composition of the CF coating was:Component % weight (dry basis) Silton AC/PC reactive clay 55 SPS diluentclay 30 Styrene-butadiene latex 15

The resulting CF product was imaged using colour forming chemicalstransferred onto the reactive clay coated surface from standardcarbonless CB paper, when pressure was applied to the 2 part-set in themanner usually associated with the usage of carbonless forms. A clearand legible image was obtained.

The resulting CF product was also magnetically imaged (encoded) with abar code using inductive magnetic writing equipment of the kindconventionally used for encoding the magnetic strips of credit cards.The resulting magnetic image was found to be readable using a magneticloop reader or suitably configured oscilloscope.

EXAMPLE 5

In this example, a carbonless CF paper was base side coated with apigment/binder layer and a magnetic layer and the resulting primer- andmagnetically-coated CF was laminated against a primer- andmagnetically-coated paper (produced as described in Example 1), using asemi-industrial laminating press.

Carbonless CF paper of 46 g/m² total weight was coated on the basesurface with 9 g/m² of pigment/binder primer coat and subsequently 5g/m² of magnetic pigment using the large-scale pilot plant coater andcoating mix formulations described in Example 1.

A pilot scale laminating press was used to laminate the magnetic surfaceof the primer- and magnetically-coated CF against a primer- andmagnetically-coated paper (as described in Example 1, except that themagnetic pigment layer was reduced to 5 g/m² on a dry weight basis. Thereduced coating weight was achieved by dilution of the coating mixsolids content). Both sheets were used with their magnetic-coatedsurfaces faced inwards. A high solids content commercial adhesive(Super-Lok 260, National Starch and Adhesives Ltd.) was used as thelaminating adhesive, applied at a coatweight range of 5-6 g/m². Standardpress conditions (speed, pressure and drying temperature) were used togenerate the laminated products.

The resulting CF product was imaged using colour forming chemicalstransferred onto the reactive clay coated surface from standardcarbonless CB paper, when pressure was applied to the 2 part-set in themanner usually associated with the usage of carbonless forms. A clearand legible image was obtained.

The resulting CF product was also magnetically imaged (encoded) with abar code using inductive magnetic writing equipment of the kindconventionally used for encoding the magnetic strips of credit cards.The resulting magnetic image was found to be readable using a magneticloop reader or suitably configured oscilloscope.

EXAMPLE 6

In this example, a pigment/binder primer-coated carbonless CF waslaminated against a pigment/binder primer-coated paper (produced asdescribed in Example 1), using a magnetically-activatable aqueouslaminating adhesive formulation.

Carbonless CF paper of 46 g/m² total weight was coated on the basesurface with 9 g/m² of primer coat using the large-scale pilot plantcoater and coating mix formulations described in Example 1.

A laboratory scale laminating press was used to laminate theprimer-coated CF to a primer-coated paper (as in Example 1) usingmagnetically-activatable aqueous laminating glue as described in Example2, except that polyvinyl alcohol was substituted for styrene-butadienelatex on a weight for weight basis. Iron oxide contents of up to 5 g/m²were achieved.

The resulting CF product was imaged using colour forming chemicalstransferred onto the reactive clay coated surface from standardcarbonless CB paper, when pressure was applied to the 2 part-set in themanner usually associated with the usage of carbonless forms. A clearand legible image was obtained.

The resulting CF product was also magnetically imaged (encoded) with abar code using inductive magnetic writing equipment of the kindconventionally used for encoding the magnetic strips of credit cards.The resulting magnetic image was found to be readable using a magneticloop reader or suitably configured oscilloscope.

EXAMPLE 7

In this example, a standard carbonless CF was laminated against aprimer-coated paper (produced as described in Example 1), using amagnetically-activatable aqueous laminating adhesive formulation. Theproduct formed is illustrated in FIG. 3, in which (1) represents sheetsof paper; (2) represents a pigment/binder primer layer; (3) represents amagnetic layer; and (4) represents a CF layer.

A base weight range of standard carbonless CF papers (57, 60 and 70g/m²) were laminated with their base sides facing inwards against theprimer face of a primer-coated paper using the methodology and apparatusdescribed in Example 6.

The resulting CF product was imaged using colour forming chemicalstransferred onto the reactive clay coated surface from standardcarbonless CB paper, when pressure was applied to the 2 part-set in themanner usually associated with the usage of carbonless forms. A clearand legible image was obtained.

The resulting CF product was also magnetically imaged (encoded) with abar code using inductive magnetic writing equipment of the kindconventionally used for encoding the magnetic strips of credit cards.The resulting magnetic image was found to be readable using a magneticloop reader or suitably configured oscilloscope. The data could bewritten and read from either side of the sheet, but better results wereobtained from the base side application of magnetic image, i.e. from theside bearing the pre-coat pigment/binder layer, than from the top sideapplication of magnetic image.

EXAMPLE 8

In this example, the magnetic laminate product was converted into acarbonless CB (coated back) product via coating following the laminationprocess. The product formed is illustrated in FIG. 4, in which (1)represents sheets of paper; (2) represents pigment/binder layers; (3)represents a magnetic layer; and (5) represents a CB layer.

Magnetic laminate product, of the type described in Example 1, wascoated with a 20% solids content CB coating formulation using alaboratory Meyer bar coater to obtain a coatweight of between 3-5 g/m²CB on a dry weight basis. The composition of the CB coating was:Component % weight (dry basis) CB microcapsules 66 Binder starch 11.5Stilt starch 22.5

The resulting CB product was used to image the reactive clay coatedsurface of standard carbonless CF paper, when pressure was applied tothe 2 part-set in the manner usually associated with the usage ofcarbonless forms. A clear and legible image was obtained.

The resulting CB product was also magnetically imaged (encoded) with abar code using inductive magnetic writing equipment of the kindconventionally used for encoding the magnetic strips of credit cards.The resulting magnetic image was found to be readable using a magneticloop reader or suitably configured oscilloscope.

EXAMPLE 9

In this example, a magnetic laminate CF product was converted into acarbonless CFB (coated front and back) product via coating following thelamination process. The product formed is illustrated in FIG. 6, inwhich (1) represents sheets of paper; (2) represents pigment/binderlayers; (3) represents a magnetic layer; (4) represents a CF layer; and(5) represents a CB layer.

Magnetic laminate CF product, of the type described in Example 5, wasbase surface coated with a 20% solids content CB coating formulationusing a laboratory Meyer bar coater to obtain a coatweight of between3-5 g/m² CB on a dry weight basis. The composition of the CB coatingwas: Component % weight (dry basis) CB microcapsules 66 Binder starch11.5 Stilt starch 22.5

The resulting CFB product was used to form the middle part of a 3-partcarbonless set interleaved between standard carbonless CB and CF papers.The middle sheet was used to receive and transmit carbonless images whenpressure was applied to the 3 part-set in the manner usually associatedwith the usage of carbonless forms. A clear and legible image wasobtained on all parts of the set.

The resulting CFB product was also magnetically imaged (encoded) with abar code using inductive magnetic writing equipment of the kindconventionally used for encoding the magnetic strips of credit cards.The resulting magnetic image was found to be readable using a magneticloop reader or suitably configured oscilloscope. Magnetic image could beapplied onto either the base side or coated side of the paper.

EXAMPLE 10

In this example, a magnetic laminate product was converted into acarbonless self-contained or autogenous product via coating followingthe lamination process. The product formed is illustrated in FIG. 6, inwhich (1) represents sheets of paper, (2) represents pigment/binderlayers; (3) represents a magnetic layer, and (6) represents anautogenous (or self-contained) layer.

Magnetic laminate product, of the type described in Example 1, wascoated with a 20% solids content self-contained coating formulationusing a laboratory Meyer bar coater to obtain a coatweight of between4-8 g/m² self-contained coating on a dry weight basis. The compositionof the self-contained coating was: Component % weight (dry basis) SiltonAC/PC reactive pigment 50 Styrene-butadiene latex 10 CB microcapsules 20Stilt starch 20

The resulting autogenous product was used to form the lower part of a 2part set with a standard 80 g/m² bond paper as the upper sheet. Whenpressure was applied to the 2 part-set in the manner usually associatedwith the usage of self-contained forms, a clear and legible image wasobtained on the self-contained surface.

The resulting autogenous product was also magnetically imaged (encoded)with a bar code using inductive magnetic writing equipment of the kindconventionally used for encoding the magnetic strips of credit cards.The resulting magnetic image was found to be readable using a magneticloop reader or suitably configured oscilloscope. Magnetic image could beapplied onto either the base side or coated side of the paper.

1. A magnetically-activatable sheet product comprising a pair oflaminated outer sheets between which is a magnetic layer comprisingmagnetically-activatable particles in a binder matrix, the outer sheetshaving sufficient opacity to mask the appearance of the magnetic layer;at least one of the outer sheets being provided on its outward facingsurface with a coating which comprises either microcapsules containing asolution of at least one chromogenic material, or dispersed dropletscontaining at least one chromogenic material in a pressure-rupturablematrix, or a colour developer composition, or both microcapsulescontaining at least one chromogenic material and also a color developer.2. A magnetically-activatable sheet product as claimed in claim 1, inwhich one outward facing surface is provided with a coating ofmicrocapsules containing a solution of at least one chromogenicmaterial, or dispersed droplets containing at least one chromogenicmaterial in a pressure-rupturable matrix, and the other outward facingsurface is provided with a coating of a color developer.
 3. Amagnetically-activatable sheet product as claimed in claim 1, whereinthe outer sheets are of paper.
 4. A magnetically-activatable product asclaimed in claim 3, wherein the outer sheets are each of a lightweightbase paper of weight 50 g m⁻² or less.
 5. A magnetically-activatablesheet product as claimed in claim 1, wherein the outer sheets are ofplastic sheet material which simulates the properties of paper.
 6. Amagnetically-activatable product as claimed in claim 1, wherein theprimer coat is formulated from conventional coating pigments as used inthe paper industry.
 7. A magnetically-activatable product as claimed inclaim 1, in which at least one of the outer sheets is provided with apigment/binder primer coat on its inward facing surface.
 8. Amagnetically-activatable product as claimed in claim 7, wherein theprimer coatweight is in the range of from 5 to 15 g m⁻².
 9. Amagnetically activatable product as claimed in claim 1, which comprisesa first outer sheet provided with a pigment/binder primer coat on itsinward facing surface, a magnetic layer comprisingmagnetically-activatable particles in a binder matrix, and a secondouter sheet being provided on its outward facing surface with a coatingwhich comprises a color developer composition.
 10. Amagnetically-activatable product as claimed in claim 1, wherein theouter sheets are substantially identical (other than the outer coatinglayer(s)) and each comprises a base sheet, a pigment/binder primer coatand a magnetic coating applied on top of the primer coat.
 11. Amagnetically-activatable product as claimed in claim 1, wherein eachouter sheet comprises a base sheet, and a pigment/binder primer coat andwherein the magnetic layer is formed by a magnetic coating applied ontop of the primer coat of one only of the outer sheets.
 12. Amagnetically activatable product as claimed in claim 1, wherein themagnetic layer is formed by a laminating adhesive applied as or justbefore the two outer sheets are brought together in a laminating pressor similar equipment.
 13. A method of storing digital magnetic data,which comprises writing digital data to a product according to claim 1,using a magnetic data writer.
 14. A method of reading digital magneticdata, which comprises writing digital data to a product according toclaim 1, and subsequently reading said data using a magnetic datareader.
 15. A magnetically-activatable sheet product as claimed in claim2, wherein the outer sheets are of paper.
 16. A magnetically-activatablesheet product as claimed in claim 2, wherein the outer sheets are ofplastic sheet material which simulates the properties of paper.
 17. Amagnetically-activatable sheet product as claimed in claim 6, whereinthe coating pigments are selected from the group consisting of calciumcarbonate, kaolin, titanium dioxide, clays, precipitated calciumcarbonate and calcined clays.